Endonuclease III (ENDOIII) initiates DNA base-excision repair by excising damaged DNA bases. Exonuclease III (EXOIII) processes the ENDOIII reaction product to prepare DNA for repair synthesis. Structures of ENDOIII and EXOIII bound to DNA will establish the mechanistic properties characteristic of an entire DNA repair pathway. We propose to determine co-crystal structures of ENDOIII and EXOIII bound to their DNA substrates. Reproducible co-crystals have been successfully flash-frozen and low-resolution data collected on our local X-ray source. Test shots indicate that synchrotron radiation is key to obtaining the high-resolution data required to decipher the structural bases of DNA repair by these essential enzymes.